1. Field
Embodiments of the present disclosure relate to a color image forming apparatus and a control method thereof, and more particularly, to a color image forming apparatus capable of detecting a position of a transfer belt where a toner image developed on a photoconductor is transferred and transferring the toner image from the photoconductor to the transfer belt and a control method thereof.
2. Description of the Related Art
In general, a color image forming apparatus is divided into a multipass scheme, which forms a color image by rotating a signal photoconductor several times, and a single pass scheme, which forms a color image by rotating each of a plurality of photoconductors once.
A color image forming apparatus using the multipass includes a transfer belt which is configured to form a primary transfer image by overlapping different color toner images formed on a photoconductor, and to transfer the primary transfer image to a recording medium.
In this case, the plurality of toner images are separately formed on the photoconductor by corresponding developing apparatuses and the toner images are individually transferred from the photoconductor to the transfer belt such that one toner image is disposed on top of another toner image. In order to obtain a uniform transfer image having less color deviation among different toner images colors when transferring the toner images to the transfer belt, a transfer start position of each toner image needs to be precisely controlled.
To this end, the color image forming apparatus includes a transfer position sensing apparatus configured to detect a transfer position of a transfer belt where each toner image formed on the photoconductor is transferred.
The transfer position sensing apparatus includes a position sensing hole, which is formed on a transfer start position of the transfer belt and serves as a position indicator, and a detection sensor, which includes a light emitting portion and a light receiving portion that are disposed along the moving path of the position sensing hole opposite each other with respect to the position sensing hole. The sensor emits a predetermined light from the light emitting portion and detects the transfer start position of the transfer belt according to whether light is received by the light receiving portion. That is, if the position sensing hole of the transfer belt, which is driven in a caterpillar manner, is disposed in a path through which light passes, the light emitted from the light emitting portion penetrates the position sensing hole and reaches the light receiving portion. Sensing such a state, the transfer start position of the transfer belt is detected.
The color image forming apparatus having such a transfer position sensing apparatus starts rotating upon generation of a print command, and the sensor generates a sensing signal through the position sensing hole formed on the transfer start position of the transfer belt.
As the sensor generates a sensing signal, the control unit determines a position of the transfer belt corresponding to the position sensing hole as a transfer start position of a first color toner image, for example, a transfer start position of a yellow toner image, and controls a laser scanning unit (referred to as ‘LSU’) and a yellow color developer such that a yellow toner image is formed on a position of the photoconductor corresponding to the transfer start position of the first color toner image.
As a result, an electrostatic latent image corresponding to a yellow toner image is formed on the photoconductor by a laser beam which is emitted from the LSU according to an image signal input from a personal computer, and a yellow toner is attached to the yellow electrostatic latent image formed on the photoconductor by a developing roller of the yellow color developer, so that a visible yellow toner image is developed.
Sequentially, when the transfer start position of the transfer belt for the yellow toner image reaches a transfer nip between the transfer belt and the photoconductor, a transfer bias voltage having a predetermined potential and an opposite polarity to that of the yellow toner is applied to a transfer roller. As a result, the yellow toner image formed on the photoconductor is transferred to the transfer belt by the transfer bias voltage and pressure of the transfer roller.
Thereafter, if the sensor generates a sensing signal through the position sensing hole again, the second toner image, a magenta toner image, for example, is formed on the photoconductor by the LSU and by a magenta color developer (similar to the method of forming the yellow toner image), and the magenta toner image is transferred to the yellow toner image on the transfer belt by a transfer bias voltage and by the pressure of a transfer roller when the transfer start position of the transfer belt reaches the transfer nip between the transfer belt and the photoconductor.
Similar to the method of forming the yellow toner image and the magenta toner image, a third toner image and a fourth toner image, for example, a cyan toner image and a black toner image, are formed on the photoconductor (by the LSU, a cyan color developer and a black color developer), and are transferred on the yellow toner image and on the magenta toner image.
However, such a conventional color image forming apparatus determines the transfer start position of the transfer belt for each toner image only based on a sensing signal, which is generated when the sensor detects either a front end or a rear end of the position sensing hole.
That is, since the position sensing hole is detected only based on the sensing signal either from a front end or a rear end of the position sensing hole, so noise may hinder a precise detection of the position sensing hole. Therefore, in order to precisely determine whether the position sensing hole is sensed, another sensing signal is needed, and thus the transfer belt needs to perform two rotations.
A transfer start point of time, at which a first color toner image (for example, a yellow toner image) starts to be transferred to the transfer belt, is determined according to when a print command is generated and whether the position sensing hole of the transfer belt is sensed. Accordingly, even if the print command is generated, the LSU is on hold without forming a yellow electrostatic latent image on the photoconductor until a sensing signal is generated through the position sensing hole.
In this case, if the detection of the position sensing hole is delayed, the overall print time is increased.
For example, in the case that a rotation period of a transfer belt is 3.75 seconds, if the transfer belt performs two rotations, a waiting time that a LSU waits to form a yellow electrostatic latent image is 7.5 seconds at the maximum. Accordingly, an electrostatic latent image forming time, at which a yellow electrostatic latent image is formed on the photoconductor, and a transfer start time, at which the yellow electrostatic latent image is transferred to the transfer belt, are delayed by the waiting time, so that the overall print time is increased.